Method for production of metal skin layer particles with controllable layer thickness

ABSTRACT

A method of depositing at least one metal skin layer on a metal nanoparticle core is disclosed. The first step of the method is selecting a metal to deposit. Then, the metal nanoparticle core is dispersed in an electrolyte solvent to form a liquid mixture. Next, a hydrogen containing gas is bubbled through the liquid mixture to form a layer of adsorbed hydrogen atoms on the surface of the metal nanoparticle core. Finally, the selected metal is added to the liquid mixture to form a metal skin layer on the metal nanoparticle core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No. 62/038,443 filed on Aug. 18, 2014 the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to producing metal skin layer nanoparticles. The present invention further relates to a method for producing metal skin layer nanoparticles having uniform and controlled layer thickness, which are adjustable from sub-monolayer to multiple layers.

BACKGROUND OF THE INVENTION

Metal nanoparticles have important uses in many applications, such as in industry and in research. For example, metal nanoparticles, which can catalyze chemical, electrochemical, and/or photochemical reactions, are used to promote reaction kinetics. Metal nanoparticles can also exhibit extraordinary optical, magnetic, thermal, and electrical properties. These extraordinary properties enable their application in many fields including imagining, sensing, information storage, biology, and medicine. Depending upon the application, the form used for the metal nanoparticles can be quite different.

For instance, metal nanoparticles are often put on a support when being used as a catalyst. Catalysis is primarily a surface phenomenon and therefore large surfaces can be obtained by making the metals into tiny particles. The use of a support is for separation and stabilization of the metal nanoparticles from agglomeration and sintering. The creation of large specific surfaces is especially important for precious metals. The use of a miniscule amount of precious metal nanoparticles on a support can improve the utilization of the precious metal while at the same time decreasing their overall usage. In another instance, unsupported metal nanoparticles are often used when their optical properties are needed and applied.

The driving force to produce metal skin layer nanoparticles with controlled thickness is multifold. First, for precious metals, minimization of their usage is always under pursuit for cost reasons. By making skin layer nanoparticles in which precious metals are only located on the particle surface layer/layers, and having less expensive metals serve as the particle core; the precious metal utilization is maximized while reducing cost. Second, for non-precious metals, it is often that their catalytic, optical, and electrical properties can be improved when one thin layer of these metals are put on other metallic substrates. By making skin layer nanoparticles in which these metals consist of the surface layer/layers, and some other metal makes up the core, the property of the particles can be improved. Finally, when unstable metal nanoparticles which have important applications are used, it is essential to protect the particles from degradation. By depositing skin layers of more stable metals onto the unstable metal particle surface, the stability of these nanoparticles is improved.

Despite the many advantages of metal skin layer nanoparticles, there is currently not a production method for facile and scalable production at a low cost. Electrochemical underpotential deposition (UPD) methods have been developed for making metal skin layer nanoparticles. UPD is a phenomenon of electrodeposition of a species (typically reduction of a metal cation to a solid metal) at a potential less negative than the equilibrium (Nernst) potential for the reduction of this metal. The equilibrium potential for the reduction of a metal in this context is the potential at which it will deposit onto itself. However these methods have obvious limitations.

Firstly, the UPD methods cannot be easily scaled up. An electrode is needed in these methods and the production rate is determined by the surface area of the electrode. Thus, the production rate is largely limited because the electrodes cannot be made large enough. Secondly, the method is applicable only to a limited combination of core and skin layer metals. Most metals are not electrochemically stable under the reaction conditions and are thus not appropriate to serve as particle cores. Finally, the support materials have to be electron conductive and the production cost is high because of the complex electrochemical equipment needed. The potential of the electrode needs to be precisely controlled, which can only be done with complex equipment.

An electroless deposition method for depositing a second metal on the surface of core metal nanoparticles has also been developed. In this method, a reducing reagent and skin-layer forming metal salts are added to a dispersion of core metal nanoparticles. The reduced metal atoms tend to grow on the core metal particle surfaces which lead to formation of core shell-like structures. However, this method has a lack of good control over the metal deposition onto the core metal particles. It is often that some metal core particles are deposited with thick metal layers while some other metal core particles still have bare surfaces because any slight differences among core metal particles can alter metal deposition behavior. The deposited layers are often not uniform in thickness even on the same particle, with some parts having multiple layers while the other parts have no layer at all.

Therefore, there is a need in the art for a method for producing metal skin layer nanoparticles wherein the metal skin layer nanoparticles have uniform and controlled layer thickness; wherein the metal skin layer nanoparticles are adjustable from sub-monolayers to multiple layers; and wherein the method can be readily scalable for mass production with low production costs.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core comprising the steps of: (a) selecting a metal to deposit; (b) dispersing the metal nanoparticle core in an electrolyte solvent to form a liquid mixture; (c) bubbling a hydrogen containing gas through the liquid mixture to form a layer of adsorbed hydrogen atoms on the surface of the metal nanoparticle core; and (d) adding the selected metal to the liquid mixture to form a metal skin layer on the metal nanoparticle core.

In a second embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in the first embodiment, wherein the selected metal is incapable of adsorbing and dissociating hydrogen molecules (M_(I)).

In a third embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in either the first or second embodiment, wherein the selected metal is capable of adsorbing and dissociating hydrogen molecules (M_(C)).

In a fourth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through third embodiments, wherein prior to step (d) the method further comprises the steps of: adding a metal ion or metal ion complex (M_(R)) to the liquid mixture wherein M_(R) is reduced by the adsorbed hydrogen atoms to form a layer of M_(R) on the metal nanoparticle core; and purging the liquid mixture of all excess hydrogen

In a fifth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through fourth embodiments, wherein the entire method is repeated multiple times so as to deposit multiple layers of M_(C).

In a sixth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through fifth embodiments, wherein prior to step (d) the method further comprises the steps of: adding a MR to the liquid mixture wherein MR is reduced by the adsorbed hydrogen atoms to form a layer of MR on the metal nanoparticle core; purging the liquid mixture of all excess hydrogen; and adding a M_(C) to the liquid mixture.

In a seventh embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through sixth embodiments, wherein every step prior to step d is repeated multiple times so as to deposit multiple layers of M_(C), and then once step d occurs; the added M_(I) will replace the multiple layers of M_(C) to form multiple layers of M_(I).

In an eighth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through seventh embodiments, wherein M_(I) is selected from the Group 11 through Group 15 metals of the Periodic Table.

In a ninth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through eighth embodiments, wherein M_(C) is selected from the Group 3 to Group 10 metals of the Periodic Table.

In a tenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through ninth embodiments, wherein the metal nanoparticle core is selected from the Group 3 to Group 10 metals of the Periodic Table.

In an eleventh embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through tenth embodiments, wherein the electrolyte solvent is selected from any solvent capable of dissolving metal salts.

In a twelfth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through eleventh embodiments, wherein the electrolyte solvent is selected from the group consisting of water, methanol, ethanol, propanol, organic electrolytes and ionic liquids.

In a thirteenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through twelfth embodiments, wherein M_(R) is selected from M_(R)'s that can undergo a reduction reaction wherein the reduced form of the M_(R) is a metal in solid form.

In a fourteenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through thirteenth embodiments, wherein M_(R) is selected from M_(R)'s that can undergo a reduction reaction wherein the reduced form of the M_(R) is a metal in solid form.

In a fifteenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through fourteenth embodiments, wherein the hydrogen containing gas is selected from pure hydrogen gas or a mixture of hydrogen gas with other gases selected from nitrogen, argon, carbon dioxide, and carbon monoxide.

In a sixteenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through fifteenth embodiments, wherein the temperature of the liquid mixture is less than about 100° C.

In a seventeenth embodiment, the present invention provides a method of depositing at least one metal skin layer on a metal nanoparticle core as in any of the first through sixteenth embodiments, wherein the pH of the liquid mixture is between about 0 and about 14.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

This invention relates to a new method for producing metal skin layer nanoparticles. The method is able to produce metal skin layer nanoparticles with uniform and controlled layer thickness, adjustable from sub-monolayer to multiple layers. In some embodiments, the metal skin layer is formed from metal ions that in metal form are incapable of adsorbing and dissociating hydrogen molecules (M_(I)). In yet other embodiments, the metal skin layer is formed from metal ions that in metal form are capable of adsorbing and dissociating hydrogen molecules (M_(C)).

The method is applicable for producing a broad range of metal skin layer nanoparticles with controlled layer thickness. The metal nanoparticle core can be selected from the Group 3 to Group 10 metals of the Periodic Table, particularly, but not limited to platinum, iridium, palladium, rhodium, ruthenium, nickel, cobalt, iron, tungsten, and rhenium. The selection of the metal nanoparticle core does not limit the practice of this invention.

To deposit a single metal skin layer on a metal nanoparticle core, a metal ion to be deposited on the metal nanoparticle core must be selected. In some embodiments of the present invention, the metal skin layer is formed from metal ions that in metal form are incapable of adsorbing and dissociating hydrogen molecules (M_(I)). These M_(I) metals are incapable of adsorbing and dissociating hydrogen molecules because they do not have strong electronic interaction with hydrogen molecules due to saturated d-electron orbitals. M_(I) metals can be selected from the Group 11 through Group 15 metals of the Periodic Table, particularly, but not limited to copper, silver, gold, zinc, cadmium, mercury, indium, gallium, tin, lead, and bismuth. The particular M_(I) is selected based on the desired end use of the metal skin layer nanoparticle.

In yet other embodiments of the present invention, the metal skin layer is formed from metal ions that in metal form are capable of adsorbing and dissociating hydrogen molecules M_(C). These metal ions are capable of adsorbing and dissociating hydrogen molecules because they have strong electronic interaction with hydrogen molecules due to having unsaturated d-electron orbitals. M_(C) metals can be selected from the Group 3 through Group 10 metals of the Periodic Table, particularly, but not limited to platinum, iridium, palladium, rhodium, ruthenium, nickel, cobalt, iron, tungsten, and rhenium. The particular M_(C) is selected based on the desired end use of the metal skin layer nanoparticle.

Once the metal ion to be deposited on the metal nanoparticle core is selected, the next step is to disperse the metal nanoparticle core in an electrolyte solvent to form a liquid mixture. The electrolyte solvent is selected from any solvent capable of dissolving metal salts. In particular embodiments, the electrolyte solvent is selected from the group consisting of water, methanol, ethanol, propanol, organic electrolytes and ionic liquids.

Once the liquid mixture is formed, a hydrogen-containing gas is bubbled through the liquid mixture. In one embodiment, the hydrogen-containing gas is pure hydrogen gas. In other embodiments, the hydrogen-containing gas is a mixture of hydrogen gas with other gases such as nitrogen, argon, carbon dioxide, and carbon monoxide. Hydrogen is the only gas that is chemisorbed because hydrogen has the unique chemical behavior that it can dissociatively adsorb to Group 3 through Group 10 metals of the Periodic Table and generate adsorbed hydrogen atoms. The use of hydrogen mixed with other gases is only used to tune the partial pressure of hydrogen and thus the adsorption behavior of the hydrogen gas. The bubbling of the hydrogen-containing gas through the liquid mixture leads to the formation of one monolayer of adsorbed hydrogen atoms on the metal nanoparticle core. Then, if the selected metal to be deposited is an M_(I), the M_(I) ion is added to the liquid mixture and the M_(I) ion will be reduced by the adsorbed hydrogen atoms on the metal nanoparticle core and one monolayer of M_(I) will be deposited and forms a metal skin layer on the surface of the metal nanoparticle core.

However, if the selected metal to be deposited is an M_(C), then a few additional steps must take place prior to forming the metal skin layer on the surface of the metal nanoparticle core. If the selected metal is an M_(C), then, after the formation of one monolayer of adsorbed hydrogen atoms on the metal nanoparticle core, a metal ion or metal ion complex (M_(R)) is added to the liquid mixture. The selected M_(R) must be able to undergo a reduction reaction with the adsorbed hydrogen atoms on the metal nanoparticle core so that the reduced form of the M_(R) is a metal in solid form. In particular embodiments, the M_(R) is selected from the following list of redox couples [M_(R)+ne⁻

M(solid)]:

Reaction [M_(R) + ne⁻ 

 M(solid)] Zn²⁺ + 2e⁻ 

 Zn Ga³⁺ + 3e⁻ 

 Ga Cd²⁺ + 2e⁻ 

 Cd In³⁺ + 3e⁻ 

 In Tl⁺ + e⁻ 

 Tl Ga⁺ + e⁻ 

 Ga In⁺ + e⁻ 

 In Sn²⁺ + 2e⁻ 

 Sn Pb²⁺ + 2e⁻ 

 Pb Cu²⁺ + 2e⁻ 

 Cu Cu⁺ + e⁻ 

 Cu W³⁺ + 3e⁻ 

 W Ge⁴⁺ + 4e⁻ 

 Ge Bi³⁺ + 3e⁻ 

 Bi Bi⁺ + e⁻ 

  Bi BiO⁺ + 2H⁺ + 3e⁻ 

 Bi + 2H₂O Ag⁺ + e⁻ 

 Ag Hg²⁺ + 2e⁻ 

 Hg SbO⁺ + 2H⁺ + 3e⁻ 

 Sb + 2H₂O

Once the M_(R) is added to the liquid mixture, the M_(R) is reduced by the adsorbed hydrogen atoms to form a layer of the M_(R) on the metal nanoparticle core. Once there is a layer of the M_(R) on the metal nanoparticle core, then the liquid mixture is purged of all excess hydrogen-containing gas. The liquid mixture is purged of all excess hydrogen-containing gas because the excess hydrogen molecules in the liquid can continue to adsorb to the M_(C) metal layer after its deposition and reduce the M_(C) ions, leading to M_(C) over-deposition. Then, once the liquid mixture is purged of all excess hydrogen containing gas, the M_(C) ions are added to the liquid mixture and the M_(C) will be reduced by the layer of the M_(R) on the metal nanoparticle core and one monolayer of M_(C) will be deposited and forms a metal skin layer on the surface of the metal nanoparticle core.

To deposit multiple M_(C) layers on a metal nanoparticle core, take the process of dispersing the metal nanoparticle core in an electrolyte solvent to form a liquid mixture, bubbling a hydrogen containing gas though the liquid mixture, adding to the liquid mixture a M_(R), purging the system of all excess hydrogen containing gas, and adding the M_(C) to the liquid mixture; and repeat it X amount of times to deposit X amount of M_(C) layers on the metal nanoparticle core. Once the first layer of M_(C) has been deposited, the layer of adsorbed hydrogen atoms will form on top of that first layer of M_(C) and the M_(R) will be reduced by the adsorbed hydrogen atoms to form a layer of the M_(R) on top of the first layer of M_(C). Then, once the second round of M_(C) ions is added, it will replace the M_(R) on top of the first layer of M_(C) and then there will be two layers of M_(C) on the metal nanoparticle core.

To deposit multiple M_(I) layers on a metal nanoparticle core, take the process of dispersing the metal nanoparticle core in an electrolyte solvent to form a liquid mixture, bubbling a hydrogen containing gas through the liquid mixture, adding to the liquid mixture a M_(R), purging the system of all excess hydrogen containing gas, and adding M_(C) to the liquid mixture; and repeat it X amount of time to deposit X amount of M_(C) layers on the metal nanoparticle core. As stated above, once the first layer of M_(C) has been deposited, the layer of adsorbed hydrogen atoms will form on top of that first layer of M_(C) and the M_(R) will be reduced by the adsorbed hydrogen atoms to form a layer of the M_(R) on top of the first layer of M_(C). Then, once the second round of M_(C) ions is added, it will replace the M_(R) on top of the first layer of M_(C) and then there will be two layers of M_(C) on the metal nanoparticle core.

Once the process has been repeated X amount of time to deposit X amount of M_(C) layers on the metal nanoparticle core, the M_(I) ions are added to the liquid mixture and the M_(I) will be reduced by X amount of layers of M_(C) on the nanoparticle core, and the M_(I) will replace the X amount of layers of M_(C) forming a metal nanoparticle core having X amount of M_(I) skin layers.

Whether depositing a single layer of M_(I), a single layer of M_(C), or multiple layers of either M_(I) or M_(C) on the nanoparticle core, the temperature of the liquid mixture should be less than about 100° C. and the pH of the liquid mixture should be between about 0 and about 14.

In light of the foregoing, it should be appreciated that the present invention significantly advances the art by providing a method of depositing at least one metal skin layer on a metal nanoparticle core that is structurally and functionally improved in a number of ways. While particular embodiments of the invention have been disclosed in detail herein, it should be appreciated that the invention is not limited thereto or thereby inasmuch as variations on the invention herein will be readily appreciated by those of ordinary skill in the art. The scope of the invention shall be appreciated from the claims that follow.

EXAMPLES Example 1

Preparation of one palladium (Pd) monolayer layer on carbon-supported platinum core nanoparticles. Platinum (Pt) nanoparticles on carbon (C) support (Pt/C) are firstly dispersed in water and stirred for mixing. The liquid mixture is bubbled with hydrogen gas for about 10 minutes. A CuSO₄ aqueous solution containing copper (Cu) ions is added and reacted for about 10 minutes, which results in formation of one Cu monolayer on the Pt core nanoparticles. The adsorbed hydrogen atoms reduce the Cu and the reduced Cu cannot adsorb hydrogen, thus the reduction stops once one Cu monolayer is generated and covers the Pt nanoparticles. The liquid mixture is then purged by nitrogen for about 20 minutes for removal of excess hydrogen dissolved in the mixture. Pd(NO₃)₂ aqueous solution containing Pd ions is added and reacted for about 10 minutes, which results in formation of Pd one monolayer on the Pt core nanoparticles. Pd has more positive reduction potential than Cu and because of this; Pd ions are reduced to Pd by Cu, which are oxidized by Pd ions to Cu ions. The formation of one Pd monolayer on the Pt/C core nanoparticles was confirmed using TEM, HRTEM, EDX elemental mapping, and EDX quantitative analyses characterizations.

Example 2

Preparation of multiple palladium layers on carbon-supported platinum core nanoparticles. The experimental procedure in Example 1 is repeated multiple times for making multiple palladium layers on Pt/C core nanoparticles. The formation of multiple palladium layers on the Pt/C core nanoparticles was confirmed using TEM, HRTEM, EDX elemental mapping, and EDX quantitative analyses characterizations. 

What is claimed is:
 1. A method of depositing at least one metal skin layer on a metal nanoparticle core comprising the steps of: a. selecting a metal to deposit; b. dispersing the metal nanoparticle core in an electrolyte solvent to form a liquid mixture; c. bubbling a hydrogen containing gas through the liquid mixture to form a layer of adsorbed hydrogen atoms on the surface of the metal nanoparticle core; and d. adding the selected metal to the liquid mixture to form a metal skin layer on the metal nanoparticle core
 2. The method of claim 1, wherein the selected metal is incapable of adsorbing and dissociating hydrogen molecules (M_(I)).
 3. The method of claim 1, wherein the selected metal is capable of adsorbing and dissociating hydrogen molecules (M_(C)).
 4. The method of claim 3, wherein prior to step d the method further comprises the steps of: a. adding a metal ion or metal ion complex (M_(R)) to the liquid mixture wherein M_(R) is reduced by the adsorbed hydrogen atoms to form a layer of M_(R) on the metal nanoparticle core; and b. purging the liquid mixture of all excess hydrogen.
 5. The method of claim 4, wherein the entire method is repeated multiple times so as to deposit multiple layers of M_(C).
 6. The method of claim 2, wherein prior to step d the method further comprises the steps of: a. adding a M_(R) to the liquid mixture wherein M_(R) is reduced by the adsorbed hydrogen atoms to form a layer of M_(R) on the metal nanoparticle core; b. purging the liquid mixture of all excess hydrogen; and c. adding a M_(C) to the liquid mixture.
 7. The method of claim 6, wherein every step prior to step d is repeated multiple times so as to deposit multiple layers of M_(C), and then once step d occurs; the added M_(I) will replace the multiple layers of M_(C) to form multiple layers of M_(I).
 8. The method of claim 2, wherein M_(I) is selected from the Group 11 through Group 15 metals of the Periodic Table.
 9. The method of claim 3, wherein M_(C) is selected from the Group 3 to Group 10 metals of the Periodic Table.
 10. The method of claim 1, wherein the metal nanoparticle core is selected from the Group 3 to Group 10 metals of the Periodic Table.
 11. The method of claim 1, wherein the electrolyte solvent is selected from any solvent capable of dissolving metal salts.
 12. The method of claim 11, wherein the electrolyte solvent is selected from the group consisting of water, methanol, ethanol, propanol, organic electrolytes and ionic liquids.
 13. The method of claim 4, wherein M_(R) is selected from M_(R)'s that can undergo a reduction reaction wherein the reduced form of the M_(R) is a metal in solid form.
 14. The method of claim 6, wherein M_(R) is selected from M_(R)'s that can undergo a reduction reaction wherein the reduced form of the M_(R) is a metal in solid form.
 15. The method of claim 1, wherein the hydrogen containing gas is selected from pure hydrogen gas or a mixture of hydrogen gas with other gases selected from nitrogen, argon, carbon dioxide, and carbon monoxide.
 16. The method of claim 1, wherein the temperature of the liquid mixture is less than about 100° C.
 17. The method of claim 1, wherein the pH of the liquid mixture is between about 0 and about
 14. 